The present invention relates to electrical signal amplification and, more particularly, to electrical signal amplification in which the error introduced into the amplified signal is effectively cancelled by a combination of positive and negative feedback to provide a distortion-free amplified output.
The classic amplifier topologies co-evolved with the development of vacuum tubes and continued through the development of bipolar transistors and, most recently, the development of MOS transistors. It was early recognized that non-linearities of the active devices in single ended amplifiers and differences between the active devices in symmetrical circuit configurations contributed to the distortion of the amplified output.
Circuits have been developed to reduce or eliminate the error and distortion components and have included error feed-forward techniques and the almost universal negative feedback technique. In the error feed-forward technique, a portion of the amplified output signal from a primary amplifier is subtracted from the input signal, leaving only the distortion component which is then amplified by a separate `error` amplifier to provide an amplified distortion signal that is then combined with the output of the primary amplifier to effect additive cancellation between the amplified distortion from the error amplifier and the distortion component of the primary amplified signal. While it is possible that all amplifier error can be completely nulled using the feed-forward technique, a separate error amplifier is required as well as very precise control of the relative gain of the separate amplifiers. Negative or degenerative feedback provides for the return of a fraction of the error-containing output signal in reverse phase to the input signal so that the reverse phase feedback effectively reduces the error introduced as the signal is propagated through the amplifier.
In comparison to the feed-forward techniques, degenerative feedback achieves generally acceptable error reduction at relatively low cost. Where high accuracy distortion-free amplification is required, the negative feedback technique has practical limitations since increased amounts of negative feedback can lead to amplifier instability and other undesirable feedback-related drawbacks including a decrease in the sonic quality of the amplifier.